The next generation wireless standard, called Fifth Generation (5G), is being designed to encompass Heterogeneous Networks (HetNets) architectures consisting of a single holistic network with Multiple Radio Access Technologies (Multi-RAT). Multiple connectivity protocols and spectrum would be managed from a common core (management system) handling both: i) traditional macro-cellular systems (such as LTE), that can provide long-range, outdoor coverage, as well as ii) low-power wireless systems with high capacity (such as Wi-Fi), that can be deployed to cater indoor traffic needs. 5G HetNets are expected to achieve ubiquitous connectivity that would guarantee Quality of Service (QoS), Quality of Experience (QoE) along with efficient use of spectrum and energy at low cost. Tightly coupled LTE—Wi-Fi networks have emerged as one of the promising solutions in the 5G era to boost network capacity and improve end user’s quality of experience. LTE/Wi-Fi Link Aggregation (LWA) and LTE WLAN Radio Level Integration with IPSec Tunnel (LWIP) are two approaches put forward by the 3rd Generation Partnership Project (3GPP) to enable flexible, general, and scalable LTE-WLAN inter-working. These techniques enable operator-controlled access of licensed and unlicensed spectrum and allow transparent access of operator’s evolved core. The most important aspect of these techniques is that they could be enabled with straightforward software upgrades and can utilize the already existing Wi-Fi networks. This article presents and motivates the design details of LWA and LWIP protocols. We also present the first NS-3 LWA and LWIP implementations over Network Simulator 3 (NS-3). In particular, this work focuses on the adaptation and concurrent usage of different NS-3 modules and protocols of different technologies to enable the support of these interworking schemes.